Absence of Ce3+ sites in chemically active colloidal ceria nanoparticles.

The catalytic performance of ceria nanoparticles is generally attributed to active sites on the particle surface. The creation of oxygen vacancies and thus nonstoichiometric CeO(2-δ) has been proposed to result in Ce(3+) sites with unpaired f electrons which can be oxidized to spinless Ce(4+) ions during catalytic reactions. We monitored the Ce electronic structure during the synthesis and catalase mimetic reaction of colloidal ceria nanoparticles under in situ conditions. By means of high-energy resolution hard X-ray spectroscopy, we directly probed the Ce 4f and 5d orbitals. We observe pronounced changes of the Ce 5d bands upon reduction of the particle size and during the catalytic reaction. The Ce 4f orbitals, however, remain unchanged, and we do not observe any significant number of spin-unpaired Ce(3+) sites even for catalytically active small (3 nm) particles with large surface to bulk ratio. This confirms strong orbital mixing between Ce and O, and the Ce spin state is conserved during the reaction. The particles show an increase of the interatomic distances between Ce and O during the catalytic decomposition of hydrogen peroxide. The redox partner is therefore not a local Ce(3+) site, but the electron density that is received and released during the catalytic reaction is delocalized over the atoms of the nanoparticle. This invokes the picture of an electron sponge.

Room-temperature photoinduced electron transfer in a Prussian blue analogue under hydrostatic pressure.

In from the cold: The Co(III)Fe(II) state of a CoFe Prussian blue analogue undergoes a Co(III)-Fe(II) →(Co(II)-Fe(III))* electron transfer at room temperature when irradiated by visible light (532 nm; see scheme). This property was confirmed using energy-dispersive X-ray absorption spectroscopy at the Co and Fe K-edges of the piezo-induced Co(III)Fe(II) state.

Photomagnetic CoFe Prussian blue analogues: role of the cyanide ions as active electron transfer bridges modulated by cyanide-alkali metal ion interactions.

X-ray absorption spectra at the Co L(2,3)-edges were analyzed by means of ligand field multiplet calculations in different states of three photomagnetic CoFe Prussian blue analogues of chemical formula Cs(2)Co(4)[Fe(CN)(6)](3.3) x 11 H(2)O, Rb(2)Co(4)[Fe(CN)(6)](3.3) x 11 H(2)O and Na(2)Co(4)[Fe(CN)(6)](3.3) x 11 H(2)O. These simulations of the experimental spectra allowed the quantification of the crystal field parameter (10Dq). This determination led us (i) to evidence different behaviors of the Co(III)(LS) and Co(II)(HS) ions in the three-dimensional structure related to their electronic configurations, (ii) to propose an approach based on the electronic density distribution along the Co-NC-Fe linkage to account for the energy position of the states implied in the switching properties of the compounds, and (iii) to explain the different photomagnetic properties observed as a function of the size of the inserted alkali cation by competing interactions between the cyanide ion and the transition metal ions within the CoFe cyanide bimetallic network on the one hand and the cyanide ion and the alkali metal ions on the other hand.